Shy, I think, from the OP, that it was a RH circuit they were to fly so the above would not have been correct on that day. What ATC, confusingly (and in a non-standard way) seemed to mean was "join downwind on the left hand side of the airfield for a right hand cct"!!!!
Yes, my understanding was that ATC wanted the OP to join from the left. Nothing wrong with that. ATC are perfectly entitled to control joining traffic - it's their job!
One reason might have been where an aircraft is about to depart and the joining aircraft is on the left side of the runway. Joining for the "correct" right hand downwind would require the joining aircraft to cross the climb out, whereas joining downwind left side wouldn't.
I can't understand why some think this is a big issue.